Although there exists a wide variety of mechanisms for pressurizing fuel in fuel injection systems, almost all fuel injectors include a spring biased needle check valve to open and close the nozzle outlet. In almost all fuel injectors, the needle valve member is only stoppable at two different positions: fully open or fully closed. Because the needle valve members in these fuel injectors are not stoppable at a partially open position, fuel injection mass flow can only be controlled through changes in fuel pressure.
Over time, engineers have come to recognize that undesirable exhaust emissions can be reduced by having the ability to produce at least three different rate shapes across the operating range of a given engine. These rate shapes include a ramp, a boot shape and square fuel injection profiles. In addition to these rate shapes, there is often a need for the injector to have the ability to produce split injections in order to further improve combustion efficiency at some operating conditions, such as at idle. While some fuel injectors have the ability to produce split injections and produce some rate shaping, a fuel injector that can reliably produce all of these rate shaping effects remains somewhat elusive.
While it has been proposed in the art that piezoelectric actuators could be employed in fuel injection systems, the use of piezoelectric actuators to directly control needle lift has proven somewhat problematic. First, this is due in part to the fact that only so much space is available within a fuel injector to place a piezoelectric crystal stack. Given the space limitations, the maximum piezoelectric deformation possible in the space available is generally on the order of less than about one hundred microns. Since typical needle valve lifts are on the order of several hundreds of microns, direct piezoelectric control of needle valve lift is not realistic without making substantial--and likely unrealistic--changes in the nozzle area of a fuel injector.
The present invention is directed to overcoming these and other problems associated with the use of piezoelectric actuators in controlling needle valve lift within fuel injectors.